As far as the CMOS process is concerned, low-cost RF-CMOS-LSIs for use in the RF (radio frequency) range have become realized with the rapid progress in RF characteristics of circuits. Significant applications thereof include, among others, transceiver ICs for portable devices, wireless LAN transceiver ICs suited for high speed, great capacity transmission, or analog/digital hybrid devices.
Since the RF characteristics attainable in the CMOS process were poor in the past, the cost of RF-CMOS circuit production was high, and the design of high-frequency circuits to be produced by the CMOS process has not been actively studied. Therefore, in the prior art high-frequency circuit designing, circuit substrates based on compound semiconductors such as GaAs have been in the mainstream. However, such substrates as GaAs-based ones are expensive from the production cost viewpoint and present problems when the wafer diameter is to be increased, and from the crystal defect controllability viewpoint, for instance, although they are excellent in RF characteristics.
On the contrary, silicon wafers are excellent in wafer diameter increasability and crystal defect controllability and are characterized by low production costs. Therefore, it is conceivable that silicon wafers be used as RF-CMOS circuit substrates but, generally, when silicon wafers with a resistance value of 10 Ωcm or below are used as circuit substrates, there is a limit to the improvement in performance characteristics of RF-ICs.
This is because, unlike digital circuits, RF circuits utilize on-chip inductor elements, highly fine capacitor elements and other analog passive elements and, in particular, the characteristics of inductors are greatly influenced by the parasitic effect of silicon wafer substrates; thus, the improvement in device performance characteristics by process refining alone is limited. For improving the device performance characteristics, it is therefore necessary to increase the resistivity of silicon wafers.
In the case of analog/digital hybrid devices, it is demanded that high-resistance silicon substrates be supplied so that the deep substrate noises produced in digital circuits may be inhibited from propagating to analog circuits.
However, silicon crystals produced by the Czochralski method (hereinafter, “CZ method”) contain oxygen therein at supersaturation levels, since the crystal growth is carried out utilizing a quartz crucible. This oxygen leads to the formation of oxygen donors called thermal donors or new donors in the heat treatment step in the device production process, causing changes in substrate resistivity as found after heat treatment in a step or steps in the device production process.
FIG. 1 is a schematic representation of the relation between oxygen donor occurrence level and wafer resistivity. In the case of low-resistance wafers with a resistivity of about 10 Ωcm, the dopant level is sufficiently high as compared with the level of generation of oxygen donors and therefore, even when oxygen donors are generated, their influence on the resistivity is insignificant. In the case of high-resistance wafers, however, the dopant level is low and the resistivity thereof is greatly influenced by the amount of oxygen donors generated.
In the case of p-type wafers, in particular, the conductivity brought about by acceptor-due positive holes disappears by the donor-due supply of electrons and the resistivity increases markedly and, upon further increase in the number of donors, an inversion occurs to n-type semiconductors and the resistivity decreases. Heating in a temperature range in which oxygen donors are readily generated is inevitably carried out as a heat treatment step in the device production process, for example in heat treatment for sintering in the wiring sintering step.
Attempts have been made to prevent the changes in resistivity as found after the device production by employing the magnetic-field-applied crystal growth method (MCZ method) to thereby reduce the solute oxygen concentration in the crystal. However, the method based on the magnetic field application leads to an increase in production costs and, in addition, reportedly, the lower limit to the solute oxygen concentration attainable with large-diameter crystals is still as high as 6×1017 atoms/cm3, indicating that there is a limit to that technology in growing extra-low-oxygen crystals. Further, as the oxygen content in crystals decreases, there arises a fear of the mechanical strength of the crystals being found deteriorated after heat treatment in the device production process.
In recent years, various proposals have been made to solve such problems. First, International Publication No. WO 00/55397 (hereinafter, “Document 1”) discloses a process comprising the steps of processing a single crystal produced by the CZ method and having a resistivity of 100 Ωcm or above and a normal oxygen content, namely an initial interstitial oxygen (solute oxygen) concentration of 10-25 ppma (7.9×1017 to 19.8×1017 atoms/cm3 [Old ASTM]) to wafers, and subjecting the wafers to oxygen precipitation treatment to lower the residual interstitial oxygen concentration to 8 ppma (6.4×1017 atoms/cm3 [Old ASTM]) or below. The wafers obtained in this way allegedly show no decrease in resistivity upon heat treatment in the device production process as a result of inhibited oxygen donor generation.
Japanese Patent Application Publication No. 2000-100631 (hereinafter, “Document 2”) discloses an invention concerning the requirements to be met by high-resistance wafers having a resistivity of 100 Ωcm or above which are to be subjected to the so-called DZ-IG treatment mentioned above as well as the treatment conditions. According to this invention, like the invention described in Document 1, each wafer has an interstitial oxygen concentration of 8 ppm or below at every part of the wafer, has a DZ (denuded zone) in the vicinity of the surface and an oxygen precipitate layer in the bulk portion and, in addition, has a transition zone of 5 μm or below in width between the DZ and oxygen precipitate layer.
According to the methods proposed in Documents 1 and 2 cited above, general-purpose silicon crystals having a high oxygen concentration are used and, therefore, the production cost can be cut, and the residual oxygen concentration is reduced by the subsequent heat treatment for oxygen precipitation. Therefore, the silicon wafers obtained have a reduced residual oxygen concentration, so that the generation of oxygen donors in the heat treatment step in the device production process can be effectively inhibited. Further, allegedly, an improvement in gettering capacity can be expected by causing a high density of oxygen precipitate defects to be formed inside of the wafer (in the bulk portion) in view of the oxygen precipitation heat treatment carried out to reduce the residual oxygen concentration.
As described above, the methods proposed in Documents 1 and 2 cited above are the methods which comprise subjecting high-resistance wafers with a high oxygen concentration, after the manufacture thereof, to a long duration heat treatment for oxygen precipitation to thereby form a high density of oxygen precipitates and sufficiently reduce the residual oxygen concentration. However, it is not always easy to reduce the residual oxygen concentration to a level of 8 ppma or below by selecting the heat treatment conditions. In addition, some problems arise thereby.
A first problem is that the mechanical strength of wafers is markedly deteriorated because the residual oxygen concentration in wafers is reduced sufficiently, or in other words, the residual oxygen concentration is reduced excessively. Thus, slip dislocations originating from wafer supporting sites, for instance, during heat treatment are immobilized by oxygen and, as a result, the slip lengths may decrease with the increase in oxygen concentration (see for example M. Akatsuka et al., Jpn. J. Appl. Phys., 36 (1997), L1422).
Further, oxygen precipitates serve as a factor giving strength to wafers and, while the thermal stress or tare stress is low, they can prevent the movement of slip dislocations and thereby increase the strength. When such stress increases, oxygen precipitates themselves serve as sources of slip dislocations, possibly causing a decrease in strength and/or wafer warping (see for example K. Sueoka et al., Jpn. J. Appl. Phys., 36 (1997) 7095).
A second problem is an increase in production cost as a result of long-duration heat treatment for oxygen precipitation. For reducing the residual oxygen concentration in the bulk portion, it is necessary to cause the formation of oxygen precipitates at a high density; hence it becomes necessary to carry out a long-duration heat treatment at elevated temperatures. Accordingly, the oxygen precipitation heat treatment causes an increase in production cost. Thus, even when the costs of production of initial wafers can be cut, the price of final product wafers cannot but increase.
A third problem is concerns about heavy metal contamination in the heat treatment furnace as resulting from the oxygen precipitation heat treatment. According to the results of investigations made by the present inventors, the heat treatment process proposed in Document 2 cited above requires at least 17 hours and extends to 47 hours of heating at the most. In view of the necessity of such a long-duration oxygen precipitation heat treatment, the risk of heavy metal contamination in the heat treatment furnace cannot be denied.